A nuclear in-reactor inspection system that uses an underwater vehicle to perform inspection tasks on structures inside a nuclear reactor has been known. The underwater vehicle is generally connected to a control device installed off the water with a cable that is used to receive power supply from and to perform signal transmission and reception with the control device. However, when the underwater vehicle is used alone to conduct inspections, the cable may be caught on the structures and therefore may obstruct movement of the underwater vehicle. As a measure to counter the problem, Japanese Unexamined Patent Application Publication No. 2008-296875 (hereinafter JPA 2008-296875) discloses a cable assist device that carries cable handling equipment and is coupled between an underwater vehicle and a control device. In JPA 2008-296875, cable tension, which is calculated from detection results obtained by a cable-tension detection unit of the underwater vehicle, and cable tension, which is calculated from detection results obtained by a tension detection unit of the assist device, are used to detect if the cable is caught on the structures.
However, the system in JPA 2008-296875 is capable of detecting the cable caught on the structures based on changes in the cable tension on the underwater vehicle side and the cable tension on the assist device side, but does not take the effects of external force generated from the cable of the underwater vehicle under regular navigation into consideration, and the external force of the cable may make it difficult for the underwater vehicle to travel.